Nanostructure, Nanochemistry and Grain Boundary Conductivity of Yttria-doped Zirconia.

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1 Solid State Phenomena Vol. 106 (2005) pp Online available since 2005/Sep/15 at (2005) Trans Tech Publications, Switzerland doi: / Nanostructure, Nanochemistry and Grain Boundary Conductivity of Yttria-doped Zirconia. A. Rizea, J.M. Raulot, C. Petot, G. Petot-Ervas, G. Baldinozzi Structures, Propriétés et Modélisation des Solides, UMR 8580 CNRS-Ecole Centrale Paris, Grande voie des vignes, Châtenay-Malabry, France. Keywords: Nanostructured materials, transport properties, ionic conductivity. Abstract. This work was directed at a comprehensive study of the role of the nanostructure and nanochemistry on the transport properties of yttria-stabilized zirconia. Alumina additions lead to a decrease of σ gb when the samples have clean grain boundaries, while σ gb goes through a maximum in samples having glassy grain boundaries. The differences were attributed to the strong interaction between Al 2 O 3 and SiO 2 impurities leading to a glassy phase depletion at the grain-boundaries, due to a change in wettability. Moreover, XPS analyses show that Si and Y segregate near these interfaces according to a kinetic demixing process, explaining why a faster cooling rate after sintering has a beneficial effect on σ gb. Introduction Yttria doped zirconia is currently the most widely used solid electrolyte. In this work, experiments were performed to understand the influence of the nanostructure and of interface nanochemistry on the transport properties of this oxide in order to decrease device operating temperatures. Experimental Samples were sintered from various powder batches [1-4]. Commercial submicronic 9.9 mol% Y 2 O 3 doped-zirconia powder was used for Z C samples. The main impurity is SiO 2 (0.42 wt %). Z F samples (9 mol% Y 2 O 3 ) were sintered using powder prepared by freeze-drying. The main impurity is also SiO 2 (~1.0wt% in A samples and 1.6 wt % in B samples). The powders were hydrostatically pressed at 2000 or 4000 bar and sintered at temperatures between 1250 and 1600 C. The electrical conductivity was measured by complex impedance spectroscopy [1-4]. The oxygen diffusion coefficient was determined by the electrochemical method [5,6]. Results Microstructural characterization and nanochemical analysis. TEM observations [1,2] show that all the triple points of samples Z C are glassy with a continuous amorphous film spreading into the adjacent grain boundaries; however, only about 20% of the triple points in samples Z F show lens-shaped glassy precipitates that do not seem to wet the adjacent grain boundaries (Fig.1). It should be noted that the microstructure is not significantly affected by less than 5mol% alumina additions. EDAX analysis shows that the main impurities are Al and Si, which concentrate at grain boundaries as a glassy phase or, eventually, as glass pockets dispersed in the grains. In the samples with no deliberately added alumina, it appears that the Si concentration is higher than the Al concentration (Al/Si ratio between 1 and 1/5). In the composites, the Al concentration was always higher than Si (Al/Si ratio close to 30). XPS analysis was performed on the fracture surface of polycrystals Z F sintered at 1600 C (5h) and then either, air quenched or cooled at 25 C min -1. They showed that cationic redistributions occur during cooling [3] and suggested that on slower cooling more Si is rejected from the grains, leading to the formation of a higher quantity of SiO 2 precipitates at the boundaries. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: , Pennsylvania State University, University Park, United States of America-03/06/14,19:09:53)

2 84 From Nanopowders to Functional Materials Electrical conductivity measurements: influence of cooling rate and Si contamination. From the results shown in Figure 1 it is clear that reducing the SiO2 content definitely has a large impact on σgb. This is of primary interest because the materials research is clearly aimed at achieving higher conductivity values (and σgb for A samples is significantly larger than σgb in B samples). These results agree with the lower values of σgb measured in the slowly cooled sample (Fig.1) and are consistent with the higher amount of glassy phase, as suggested by the XPS results. Figure 1. Influence of the cooling rate and Si contamination (~1.0 wt % in samples A and 1.6 wt % in samples B) on σgb in samples ZF and grain boundary microstructure of ZF and ZC samples. Electrical conductivity measurements: influence of the starting materials and sintering conditions. The results shown in Figure 2 are from furnace cooled samples after different sintering times. The bulk conductivity (σb) of different samples was close to the reference single crystal (9.5 mole% yttria) while grain boundary conductivity increased with sintering temperature [1-3]. Figure 2. Left panel: influence of the sintering conditions on the grain boundary conductivity of samples ZF (left panel). Right panel: comparison of the grain boundary conductivity of samples ZC (circles) and ZF (triangles) sintered at 1350 C, for 5 h. Consider σgb, ZF samples (SiO2 ~1.0 wt %) presented higher values than those observed for ZC samples less contaminated by silica (SiO2 ~0.42 wt %) (Fig. 3b). These results and the microstructural characterizations (Fig. 1) clearly show that the spatial distribution of the glassy phase at the grain boundaries plays a key role on σgb and it explains the lower values measured in ZC samples. Furthermore, σgb has the same activation energy in the different samples. According to the microstructural differences between ZF and ZC samples, this result consistently supports the idea that inter-granular conduction processes take place only via clean grain boundaries.

3 Solid State Phenomena Vol Grain boundary conductivities in YSZ alumina composites. Fig.3 shows that the grain-boundary conductivity of Z C and Z F (B) samples possesses a maximum, which occurs with higher amounts of Al 2 O 3 when the sintering temperature is decreased. Figure 3. Influence on σ gb of the amount of alumina in samples Z C (triangles) and Z F (samples A squares - and B circles -) at two sintering temperatures. These results were attributed to a change of the grain boundary wettability of the glassy phase in the presence of alumina additions due to the higher amount of Al found in the phase (see above). There is effectively a competition for σ gb between the beneficial influence of alumina, leading to a partial cleanup of the grain boundary regions, and the insulating behaviour of the alumina particles, not dissolved in the siliceous precipitates. These findings were confirmed by the results obtained with the Z F (A) polycrystals, less contaminated by Si than Z F (B) samples. The A samples show a decrease of σ gb when alumina is added (Fig.3) because the negative influence of the insulating alumina particles is no longer counterbalanced by a reduction of the glassy phase at the boundaries. It should be noted that this analysis is consistent with the conclusions of Butler and Drennan [7], who assumed that Al 2 O 3 acts as a "scavenger" for SiO 2. Oxygen diffusion coefficient. The experiments were performed by the electrochemical method [5,6]. The opposite ends of the sample, coated with the same electrode material, were inserted between reversible electrodes and subjected to different oxygen partial pressures (P I O2 and P II O2 ). The oxygen diffusion coefficient is given by the following relation : RT I D sc O = g E (1) open 4F2CO where g=l/s is the sample geometric factor, I SC =s i SC the short circuit current, C O -- the I concentration of O -- per cm 3 and E RT P = ln O2 the e m f measured at the cell terminals in open open 4F P II O 2 circuit conditions. Fig.4 shows that the values of D O (Eq.1) obtained for samples Z F sintered at 1600 C for 40 h are in good agreement with those of a reference yttria (9.5 mole %)-doped zirconia single crystal [4,5,8]. The diffusion mechanisms and the jump frequencies involved in the transport processes in these materials are then very similar. In Fig.4, the conductivity diffusion coefficient (D σ ) is also reported. D σ is calculated from total conductivity values, using the Nernst-Einstein relation and assuming a unit correlation factor (f i ) in the generalized form [9]: σ i kt Dσ = f1 ( z e) 2 (3) C i i

4 86 From Nanopowders to Functional Materials Note as shown by Kikuchi and Saito [9], that the correlation factor f i is equal to one when all the sites are equivalent (e.g. isolated defects). Figure 4. Oxygen diffusion coefficient of sample Z F sintered at 1600 C for 40 h. Comparison with the values obtained with a single crystal [5,6,8] and with those of D σ. Note that D O <D σ in the analysed Z F samples. This suggests that the sites involved in the transport processes under an applied electric field or an oxygen potential gradient are not equivalent. The results are consistent with the presence of complex defects in the investigated experimental conditions. Conclusions. This work indicates the key role played by the presence of both Si and Al impurities and by the distribution of glassy phases at the grain boundary on the transport properties of YSZ. It clearly suggests that the intergranular conduction mechanisms take place only at clean grain boundaries. Furthermore, XPS analysis also shows that Si segregates close to grain boundaries due to kinetic demixing processes. The segregation is accompanied by a greater amount of SiO 2 precipitating at the grain boundaries when the cooling rate after sintering is slower. This led to a decrease of σ gb which should be avoided with materials required for technological applications. References [1] C.Petot, M.Filal,A.Rizea, K.H.Westmacott, J.Y.Laval, C.Lacour, Microstructure and ionic conductivity of yttria-doped zirconia, J.Eur.Cer.Soc., 18, (1998) [2] A.Rizea, D.Chirlesan, C.Petot, G.Petot-Ervas Alumina influence on the microstructure and grain boundary conductivity of YSZ, Sol.St.Ionics, 146, (2002) [3] A.Rizea, C.Petot, G.Petot-Ervas, M.J.Graham; G.I.Sproule, Kinetic demixing and grain boundary conductivity of yttria-doped zirconia, Ionics, 7, (2001) [4] M.Filal, C.Petot, M.Mokchah, C.Chateau, J.L.Charpentier, Ionic conductivity of yttriumdoped zirconia and the composite effect, S. St. Ionics, 80, (1995) [5] G.Petot-Ervas, C.Petot, Electrode materials, interface processes and transport properties of yttria-doped zirconia, Ionics, 3, (1997) [6] G.Petot-Ervas, C.Petot, Experimental procedure for the determination of diffusion coefficients in ionic compounds, Sol.St.Ionics, 117, (1999) [7] E. P Butler, J.Drennan, Microstructural analysis of sintered high-conductivity zirconia with alumina additions, J.Am.Cer.Soc, 65, (1982) [8] H.Solmon, C.Monty, C.Dolin, Zr, Y and O self-diffusion in yttrium-doped zirconia, Ceramic Transactions, The American Ceramic Society, 24, (1992) [9] H.Sato, R.Kikuchi, Cation diffusion and conductivity in solid electrolytes, J.Chem.Phys., 55, 2, (1971)

5 From Nanopowders to Functional Materials / Nanostructure, Nanochemistry and Grain Boundary Conductivity of Yttria-Doped Zirconia /